Author Topic: finding short on motherboards with a shorty (with display)  (Read 50876 times)

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Offline kripton2035Topic starter

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Re: finding short on motherboards with a shorty (with display)
« Reply #150 on: March 12, 2024, 09:03:32 am »
the shorty is not a precision device. what you need to measure is two relative points to see at less than a milliohm if the resistance is lower or higher between the two points. the absolute ohm value is not important, and not to be trusted to the last digit !
 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #151 on: March 12, 2024, 10:41:27 am »
Okay, thanks for the replies. I understand that it's not a precision device but it has very good components to be one, right? Maybe there is still room for improvement.
« Last Edit: March 12, 2024, 11:03:57 am by wacd »
 

Offline kripton2035Topic starter

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Re: finding short on motherboards with a shorty (with display)
« Reply #152 on: March 12, 2024, 11:13:06 am »
I will see if I can find the good function for the devices you measured, but the current may be not constant for the measure so that's why the value is not linear compared to the resistors.
 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #153 on: March 12, 2024, 08:30:52 pm »
Improvement on the software side would be good, what else can be done to make the circuit physically better than it is now? Constant current source maybe?
 

Offline aeg

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Re: finding short on motherboards with a shorty (with display)
« Reply #154 on: March 13, 2024, 12:08:43 am »
Make sure to null out the probes before measuring shorty_measured_res, and again before making your confirmation measurements.

If that's not good enough, change "Rx = Rx * calibration_factor;" to "Rx = (Rx + 0.039) * 0.916;" for linear two-point calibration.
 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #155 on: March 13, 2024, 06:32:37 am »
I Appreciate all the inputs.

 

Offline kripton2035Topic starter

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Re: finding short on motherboards with a shorty (with display)
« Reply #156 on: March 13, 2024, 07:15:49 am »
Yes I'm confident that a constant current source would improve the device accuracy...
 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #157 on: March 13, 2024, 03:01:05 pm »
Today i tried supplying constant 50mA from a lab power supply , removing the transistor and the following resistor network.
Results are promising. Funky readings went away but need a little more special care.

Edit: i didnt supply constant current but limit the current Max at 50ma.
« Last Edit: March 13, 2024, 04:10:13 pm by wacd »
 

Offline kripton2035Topic starter

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Re: finding short on motherboards with a shorty (with display)
« Reply #158 on: March 13, 2024, 03:35:07 pm »
you could try something like this
 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #159 on: March 13, 2024, 04:35:54 pm »
 

Offline kripton2035Topic starter

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Re: finding short on motherboards with a shorty (with display)
« Reply #160 on: March 13, 2024, 04:38:56 pm »
why not, but for me not a drawer component I have around, easier to get 2 ex of some basic silicon diode ?
 

Offline Kean

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Re: finding short on motherboards with a shorty (with display)
« Reply #161 on: March 14, 2024, 04:33:10 am »
How abour this CMJA5050

https://my.centralsemi.com/datasheets/CMJA5050.PDF

Did you review the datasheet for that part?

It has a 10% tolerance, takes over 10 seconds to come into regulation, and has a TC of -0.3mA/degC.
It is not a precision part, and it also seems to need a supply of 8V or so to regulate.
 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #162 on: March 14, 2024, 05:38:22 am »
Okay, how do i limit the current to 50mA then?
These are the results when i use the lab psu limited at 0.05A its almost perfect.
 

Offline Kean

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Re: finding short on motherboards with a shorty (with display)
« Reply #163 on: March 14, 2024, 11:42:13 am »
Okay, how do i limit the current to 50mA then?
These are the results when i use the lab psu limited at 0.05A its almost perfect.

If you want precision, I would suggest changing the design to use an opamp controlled current source.  https://en.wikipedia.org/wiki/Current_source#Op-amp_current_sources

So, looking at the original Shorty schematic, R3 would be the sense resistor and selected for higher precision, and R1 would be replaced with a transistor controlled by the opamp.  R2 and the DUT become the load.  Assuming R3 stays at 1 ohm, you would need a voltage reference (the 5V or 3.3V rail may be OK) and divider to get the 50mV reference.  Opamp would have to be selected for a low offset voltage.
 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #164 on: March 14, 2024, 12:32:45 pm »
Thank you for providing information. I'll go and spend some time on this to learn it better so i can implement it , hopefully :)
 

Offline Kean

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Re: finding short on motherboards with a shorty (with display)
« Reply #165 on: March 14, 2024, 01:46:32 pm »
Thank you for providing information. I'll go and spend some time on this to learn it better so i can implement it , hopefully :)

If you don't have experience with opamps, then a much simpler but slightly less accurate solution would be using an LM317L as the current source (in place of R1).
You can get the 50mA using just the LM317L IC and a 25 ohm 1% or better resistor, and calibrate the final result in the firmware.
This will perform much better than that part you suggested earlier.

Note that 25 ohms is not a standard resistor value, so just use a 24.9 ohm, or parallel 3 x 75 or 4 x 100 ohm.  Or some other combination like 10 + 15 ohm in series.
 
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Offline kripton2035Topic starter

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Re: finding short on motherboards with a shorty (with display)
« Reply #166 on: March 14, 2024, 02:13:52 pm »
if you feed 50mA constant, I'm afraid you can't measure resistors above 0.5Ω
it would be better to stay with 30mA.


 
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Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #167 on: March 14, 2024, 04:53:46 pm »
Oktay, Lm317L looks easy enough , i'll give it a go.
 

Offline wacd

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Re: finding short on motherboards with a shorty (with display)
« Reply #168 on: March 26, 2024, 05:24:11 pm »
Hello again. I've been playin with the calibration settings and i think i finally arrived at a point that the results satisfy me.
Lower values are absolutely on point. It reads slightly lower value above 1ohm.
Ordered a couple mcp3421's , waiting for their arrival. The one i've set up on breadboard will stay for further tinkering.
And i made this pcb ordered from oshpark, hope it works as i'd like it to.
 

Offline CaptainBucko

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Re: finding short on motherboards with a shorty (with display)
« Reply #169 on: May 01, 2024, 03:13:16 am »
PCBs for JDobrey's Shorty Pen have arrived from JLPCB.
5 x PCBs for $4.00USD delivered. Crazy at how low cost they can make these.
I have sourced all my components from LCSC and waiting for them to arrive, and happy to share the BOM if people want it. It's a bit different from JDobrey's which was from 2022.
2155387-0
« Last Edit: May 01, 2024, 03:16:19 am by CaptainBucko »
 

Offline eliocor

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Re: finding short on motherboards with a shorty (with display)
« Reply #170 on: May 01, 2024, 07:25:16 am »
I have sourced all my components from LCSC and waiting for them to arrive, and happy to share the BOM if people want it. It's a bit different from JDobrey's which was from 2022.

If possible, yes. TIA
 

Offline CaptainBucko

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Re: finding short on motherboards with a shorty (with display)
« Reply #171 on: May 01, 2024, 08:39:44 am »
Official Pages:
https://gitlab.com/jdobry/shortypen/
https://oshwlab.com/jdobry/shortypen

My BOM alternatives attached - Excel and PDF.
 
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Offline Ocela

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Re: finding short on motherboards with a shorty (with display)
« Reply #172 on: May 16, 2024, 09:51:05 am »
After a certain period of operation of the device Shorty Pen (frequent calibrations), malfunctions began.
1. After switching to left-handed mode and turning off the device, it is no longer possible to fully return it back.
Plus, calibration no longer works in this mode. At 8.2 Ohm the device turns off without making a sound..
The device does not turn off normally (there is no sound signal and turns off only after the button is released)
2. Also... if during calibration you press the save button with the probes open, the device freezes.
You need to turn off the device again and repeat the calibration process again. I noticed that different devices may have different glitches, but the meaning of glitches is the same - it occurs when information is written to the EEPROM.
The REASON seems to be an overflow of EEPROM or accumulation of errors in the EEPROM..
After reflashing the MK - everything is OK!
 

Offline Evi

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Re: finding short on motherboards with a shorty (with display)
« Reply #173 on: May 18, 2024, 11:27:29 am »
@kripton2035
what is the purpose of Q1 at all?
According code it is permanently ON.
 

Offline kripton2035Topic starter

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Re: finding short on motherboards with a shorty (with display)
« Reply #174 on: May 18, 2024, 12:03:28 pm »
@kripton2035
what is the purpose of Q1 at all?
According code it is permanently ON.
yes, it was connected only when measuring on the original device.
I recall it was because it had an auto shut off
here, as I use a mechanical switch to power it on, I did not need anymore.
also it should flow more current in the test probes than with the arduino outputs.
 


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